The present invention relates to a spread spectrum modulation communication apparatus and, more particularly, to a spread spectrum modulation communication apparatus which can detect or eliminate interference waves and maintain stable communication quality.
Generally, wireless communication apparatuses employing spread spectrum modulation are known for high anti-interference wave capability. Even if there exists a narrow-band interference wave within a reception band, the interference wave is spread in the spread spectrum demodulation process. The voltage of the demodulated interference wave becomes approximately 1/spread ratio (hereinafter, the spread ratio is represented by "s").
Accordingly, in comparison with a conventional narrow band system, the voltage of the anti-interference wave capability of the spread spectrum modulation type communication apparatus can be s times. FIGS. 42A to 42C show waveforms on reception in the spread spectrum modulation type communication apparatus (hereinafter, simply referred to as "communication apparatus"). FIG. 42A shows a spectrum signal waveform at a receiving terminal of the communication apparatus, FIG. 42B, a spectrum signal waveform after spread spectrum demodulation; and FIG. 42C, a spectrum signal waveform after passing through a narrow band-pass filter and before data demodulation.
However, if the communication apparatus and the conventional narrow band system share the same frequency, narrow band interference occurs so often. The interference wave power may become very large depending on the setting position of a receiving apparatus and exceed the anti-interference wave capability of the apparatus. In such case, it is impossible to maintain stable communication quality.
FIGS. 43A to 43C show signal waveforms in the above case. More specifically, FIG. 43A shows a spectrum signal waveform at the receiving terminal of the communication apparatus; FIG. 43B, a spectrum signal waveform after spread spectrum demodulation; and FIG. 43C, a spectrum signal waveform after passing through the narrow band-pass filter and before data demodulation.
Apparently, a non-negligible interference wave remains in a signal after spread spectrum demodulation and filtering by the narrow band-pass filter (FIG. 43C).
If the communication apparatus is used for digital data wireless communication, a communication protocol including an error correction procedure and a retransmission procedure is required for maintaining communication quality. For this reason, the data is transmitted in packet mode and the protocol is realized by, e.g., HDLC (High level Data Link Control) procedure.
FIGS. 44 and 45 show an example of the retransmission procedure according to the HDLC procedure. More specifically, FIG. 44 shows a rejection procedure (Rej) and FIG. 45 shows a selective rejection procedure (SRej).
In FIG. 44, a data transmitting apparatus (hereinafter, referred to as "transmitter") transmits a data packet (I), and one portion having a frame number "1" and a packet number "5" (I.sub.1-5) fails to reach the data receiving apparatus (hereinafter, referred to as "receiver").
The receiver waits until time set to reception confirmation timer T2 becomes "over", then transmits an RR (receive ready) signal and starts retransmission timer T1. Thereafter, the receiver waits for a response from the transmitter.
On the other hand, the transmitter transmits the I.sub.1-5 in which a poll bit (P: reception confirmation request bit) is set to "1" and starts the timer T1, thereafter, waits for confirmation till time set to the timer T1 becomes "over".
However, in FIG. 44, the transmitter cannot receive the RR signal from the receiver due to a communication condition, the time set to the timer T1 at the transmitting side becomes "over". Accordingly, the transmitter clears the timer T1 and restarts it, and at the same time transmits an RR signal in which the poll bit is set to "1". If there is no response from the receiver until the time set to the timer T1 becomes "over" again, the transmitter repeats the above retransmission up to N.sub.2 times.
If the communication condition is reactivated and communication between the apparati becomes possible, the receiver compares a transmission status variable (T.sub.s : value indicating a confirmed data packet number from the transmitter+1) indicated by RR signal and a reception status variable (Rs: value indicating a data packet number correctly received by the receiver). If the variables T.sub.s and R.sub.s are different, the receiver requests retransmission, i.e., transmits a Rej command in which the reception status variable R.sub.s is set as a reception sequence number to the transmitter.
The transmitter receives the Rej command and retransmits the I.sub.1-5 data corresponding to the reception sequence number included in the Rej command. As the poll bit of the data is "1", the transmitter waits for an RR signal having a variable R.sub.s corresponding to the T.sub.s from the receiver. Upon receiving the RR signal, the transmitter restarts to transmit the next data (I).
Next, the SRej (Selective Rejection) procedure will be described with reference to FIG. 45. In FIG. 45, only procedures different from the Rej procedure will be explained. As shown in FIG. 45, the poll bit (P) is not set to "1", i.e., the reception confirmation is not made, and the receiver does not receive data I.sub.1-3 but correctly receives the next data I.sub.1-4.
The receiver transmits a SRej command in which a parameter indicating a reception number of the not-received data I.sub.1-3 to request retransmission. At this time, if the communication condition is fine, the transmitter that receives the SRej command during transmission (I.sub.1-4) retransmits the data I.sub.1-3 immediately after the transmission of the data I.sub.1-4. Thereafter, the transmitter continues transmission in the initial transmission order.
In the above-described communication according to the HDLC procedure, the lack of data can be retransmitted as far as the degradation of communication condition is recovered in a short period. However, if the degradation of communication condition continues over seconds, the retransmission should be repeated up to the maximum number of retrying times set in the protocol as shown in FIG. 46. As a result, the data link might be disconnected.
Such repetition of retransmission up to the maximum number of times, the disconnection and re-establishment of the data link lower the throughput. In addition, the transmission of radiowaves in non-communicable status has ill effects on the radiowave propagation condition, which further influences the other wireless communication systems, causing problems in common use of radiowave resources.
In a system having only the Rej retransmission procedure, the transmitter sequentially performs data transmission up to an outstanding number (maximum number of times within which transmission without reception confirmation is possible). In a system which performs time division bidirectional communication, if a lack of packet occurs when the number of transmitted packets is small, the received data from the lacked packet data to the data at the outstanding number are deleted from the receiver. Accordingly, when the transmitter retransmits the lacked data at a point where the communication condition is reactivated, the transmitter should retransmit data from the head data, thus lowering the throughput. Further, the transmission of radiowaves in non-communicable status, for a long period, ill effects the radiowave propagation condition.
In a system having a combination of the SRej and Rej procedures, the outstanding number can be set to a greater number and the lowering of throughput can be improved. However, in order to increase the outstanding number, the storage capacity should be enlarged, and the protocol control becomes complicated. The enlarged system hardware causes an increase in manufacturing costs as well as difficulty in downsizing of the apparatus.
In the transmitting apparatus for the spread spectrum modulation, if the band is spread to a fully broad band with respect to its data transfer speed, the influence of interference waves can be ignored. In this case, if the spread spectrum (SS) signal is multiplexed in the same frequency band, it does not seriously influence the spread spectrum transmission efficiency.
However, if an intense external interference wave such as a microwave from radar and a microwave oven intrudes into the receiving apparatus during the multiplexing, it lowers the transmission efficiency, since the number of available channels and transmission capacity are restricted depending upon the size of the interference wave.
In one proposed method for improving the degradation of the transmission efficiency, increasing processing gain in the spread spectrum modulation communication system is used (hereinafter, referred to as "SS communication system).
However, in this method, widening of the spread bandwidth causes a band restriction problem and difficulties in initial synchronization seizure. For this reason, the processing gain cannot be increased without limitation. Another method for directly eliminating or reducing the interference wave is desired for improving the transmission efficiency in the SS communication system.
FIG. 47 shows a demodulator in the conventional SS communication system for eliminating/reducing interference wave.
In FIG. 47, a reception signal which enters input terminal a includes communication wave d(t), interference waves U.sub.s (t) and U.sub.i (t). U.sub.s (t) is an arbitrary spread spectrum interference wave; and Ui(t), a noise component and the other SS interference wave component.
In the method employed in the modulator of FIG. 47, spread spectrum demodulator (SSDEM) 831 spread-spectrum-demodulates the SS interference wave. After the S/N ratio is raised by narrow band-pass filter (narrow BPF) 832, spread spectrum modulator (SSMOD) 833 spread-spectrum-modulates the demodulated wave and supplies the reproduced SS interference wave to a negative (-) input terminal of subtracter 834. On the other hand, delay circuit 830 matches the phase and amplitude of the reception signal to the reproduced SS interference wave and supplies the reception signal to a positive (+) input terminal of the subtracter 834. Finally, the subtracter 834 subtracts the SS interference wave from the input signal.
It should be noted that the narrow BPF 832 can be replaced with a narrow band elimination filter such as narrow band elimination filter (BEF) 835 in FIG. 48. In this case, after the SS demodulation by the SSDEM 831, the narrow BEF 835 eliminates a narrow band SS demodulated signal, and the SSMOD 833 modulates the signal to reproduce a desired signal.
FIG. 49 shows another example of the demodulator. In FIG. 49, the interference wave included in the reception signal is eliminated by the narrow BEF 835 before SS demodulation by SSDEM 836.
FIG. 50 is a block diagram showing a detailed construction of the SSDEM 836. In FIG. 50, the SSDEM 836 comprises high pass filters (HPF) 837 and 838, multiplier 839 and low pass filter (LPF) 840.
The HPF's 837 and 838 can be replaced with LPF's, if their passing characteristics are the same. Cutoff frequency f.sub.c of the HPF's 837 and 838 corresponds to a point where the energy is approximately half of the mainrobe of the SS signal generated in the transmitter (not shown).
Next, the interference wave elimination by the SSDEM 836 will be described with reference to signal waveforms in FIG. 51.
In FIG. 51, waveform (a) denotes the information signal D having only the lower component spectrum before data spread by the transmitter (not shown); and waveform (b), a SS demodulated wave D.sub.ss in which the siderobe of the signal spread by the modulator using a spread code has been eliminated.
In mid-course of the transmission of the D.sub.ss wave by the transmitter via an antenna (not shown) of the receiver to the input terminal a in FIG. 50, mixture of interference wave easily occurs in the space propagation path. In FIG. 51, waveform (c) represents the D.sub.ss wave in which an intense interference wave is mixed.
In the demodulation by the SSDEM 836, lower frequency component from the cutoff frequency of the input signal from an input terminal a ((c) in FIG. 51) is reduced by the HPF 837 to obtain a spectrum signal d(D'.sub.ss +U')
On the other hand, a spread signal P ((e) in FIG. 51) spread-modulated using the same spread code as that in the modulator of the transmitter is inputted from input terminal b. Also, lower frequency component from the cutoff frequency of the signal P is reduced to obtain a signal P' as shown in FIG. 51 (f). The signal P' and the signal d(D'.sub.ss +U') are provided to the multiplier 839 in which the d(D'.sub.ss +U') is demodulated, and a signal g(U'.sub.ss +D') is obtained. The signal g(U'.sub.ss +D') passes through the LPF 840, by which the interference wave and the spread-interference wave are eliminated, then only demodulated information signal D' ((h) in FIG. 51) can be outputted from output terminal c.
However, the above interference wave elimination method has a problem in that the noise component (noise component in the narrow BPF after the inverse-spread) remains in the demodulated information signal. Further, the conventional method is effective only if the frequency of interference wave is already-known. However, it is not effective against interference wave having an unknown frequency.
The SS communication apparatus has a variable attenuator for changing the damping rate to maintain a constant output voltage with respect to voltage variation of a reception radiowave.
FIG. 52 is a block diagram showing an intermediate frequency amplifier (IF AMP) with an automatic gain controller (AGC) which is generally used in an IF AMP. In FIG. 52, amplifiers (AMP's) 930, 932 and 933 respectively amplify an input signal. A portion surrounded by a broken line A generates a DC control voltage for controlling the damping rate of variable attenuator (variable ATT) 931. First, AMP 934 amplifies diode normal direction voltage used for envelope detection (to be described later) so that the voltage can have a fully large amplitude. Envelope detector 935 detects an envelope, then, integrator 936 converts the envelope into DC voltage, further, DC amplifier (DCAMP) 937 amplifies the DC voltage to obtain feedback loop gain. Thus, the output level variation with respect to the input level variation can be suppressed.
The AGC voltage generated in the above manner controls the variable ATT 931 which changes its damping rate to absorb the variation of the input voltage, thus a constant output voltage can be maintained. Especially, as SS modulated signals have a broad band, using an attenuator is effective. The conventional AGC varies the damping rate using e.g. PIN diode.
FIG. 53 is a block diagram showing the configuration of a conventional SS communication apparatus using the AGC in FIG. 52. This SS communication apparatus, performs half-duplex communication.
In FIG. 53, antenna 441 is used for both transmission and reception. Antenna switch (ANTSW) 462 switches over the transmission/reception of the antenna 441 by switchover signal S.sub.11. The ANTSW 462 comprises a semiconductor switch utilizing PIN diode characteristics such as a high-frequency relay.
Further, the SS communication apparatus has a band-pass filter (BPF) 442 for eliminating unnecessary radiowave included in a reception signal, low-noise RF amplifier (RF AMP) 443 which amplifies the reception signal, local oscillator (local OSC) 444 for frequency conversion, mixer 445 which multiplies the signal from the local OSC 444 and the reception signal, BPF 446 for taking an IF frequency out of the output signal from the mixer 445, variable attenuator (ATT) 463 which attenuates the output from the BPF 446, AMP 447 which amplifies the attenuated signal outputted from the ATT 463, and correlator 448 for detecting a correlation peak in a spread code with respect to the reception signal.
Further, the SS communication apparatus has a delay device 449 which delays the correlation output, mixer 450 which multiplies the relayed output from the delay device 449 and the output from the correlator 448, LPF 451 which cuts off a high-frequency zone of the mixer 450 output, envelope detector 453 which detects an envelope in the output from the correlator 448, integrator 454 which integrates the detected signal outputted from the envelope detector 453, clock regenerator 456 which regenerates a clock from the detected signal, comparator 452 which judges the level of the LPF 451 output, and comparator 455 which judges the level of the integrator 454 output.
Further, the SS communication apparatus has reference oscillator (reference OSC) 457 for transmission, PN code generator 458 which adds a PN code to a transmission signal, mixer 459 which multiplies the transmission signal and the reference-oscillated frequency, mixer 460 which multiplies the frequency signal from the local OSC 444 and the reception signal, and RF amplifier (RF AMP) 461 which amplifies the transmission signal and outputs the signal to the ANTSW 462.
Upon transmission in the SS communication apparatus having the construction as described above, a controller (not shown) transmits an ON signal (S.sub.12) for the RF AMP 461 and a switchover signal S.sub.11 to switchover the ANTSW 462 to the antenna 441 side.
The mixer 459 modulates transmission data with a random code ((b) in FIG. 54) outputted from the PN code generator 458 and a carrier signal ((a) in FIG. 54) from the reference OSC 457 to obtain a spread signal ((c) in FIG. 54).
The mixer 460 converts the spread signal with the reference signal from the local OSC 444 into an RF frequency. The RF AMP 461 amplifies the RF frequency and supplies the amplified RF frequency to the antenna 441 via the ANTSW 462.
Upon reception, the controller transmits a switchover signal S.sub.11 to switchover the antenna 441 by the ANTSW 462 to the reception side. The reception signal passes through the RF PBF 442, where unnecessary waves are eliminated and only a desired signal is outputted.
The RF AMP 443 amplifies this signal and inputs the amplified signal into the mixer 445, which converts the signal from the RF AMP 443 with a predetermined frequency signal from the local OSC 444 into an IF frequency (hereinafter, referred to as "IF signal").
Next, only the IF frequency component is taken out of the IF signal in the BPF 446, and the ATT 463 attenuates the IF frequency component signal in accordance with AGC voltage to supply a constant voltage IF signal to the correlator 448. In the correlation output from the correlator 448 ((a) in FIG. 55), the correlation output peak interval is proportional to transmission data speed (T.sub.1). This correlation output is supplied to the envelope detector 453 and the delay device 449. The delay device 449 delays the correlation output by one bit of the transmission data ((b) in FIG. 55), and the mixer 450 multiplies the correlation output before delay and the delayed correlation output to obtain demodulation output ((c) in FIG. 55).
The demodulation output passes through the LPF 451, in which high-frequency component is eliminated. The comparator 452 judges the level of the demodulation output, and finally, reception data ((d) in FIG. 55) can be obtained. Note that as it is understood from the above description, the demodulator of this SS communication apparatus is a differential demodulator.
On the other hand, the envelope detector 453 detects an envelope out of the supplied correlation output and the envelope detection output ((a) in FIG. 56) is obtained. This detection output is inputted into the clock regenerator 456 in which clock component is extracted, and finally, reception clock ((c) in FIG. 56) is obtained.
Further, the envelope detection output is also supplied to the integrator 454 which converts the detection output into DC voltage corresponding to the peak level ((c) in FIG. 56). The DC voltage is used as the AGC voltage for controlling the damping rate of the ATT 463. The output of the integrator 454 is supplied to the comparator 455, which determines whether or not there is a desired voltage level, i.e., whether or not there is a reception signal, and outputs the integrator output as a carrier sense signal to the controller.
However, in the above SS communication apparatus, if the interference wave mixes in a reception signal, the effect of the interference wave appears in the correlation output (e.g., (a) in FIG. 57), degrading the correlation output S/N ratio.
Further, a signal other than the correlation output, e.g., the input voltage of the correlator, may directly appear as the correlation output. If AGC voltage is generated using such correlation output in the conventional manner, the envelope detection output may include its noise component ((b) in FIG. 57). Accordingly, the AGC voltage whose level corresponds to the correlation output noise ((c) in FIG. 57) may be generated, which becomes an impediment of exact automatic gain control.
For bidirectional communication by this SS communication apparatus, a full duplex system for simultaneous transmission/reception and a half duplex system for alternative transmission/reception can be considered. The former system comprises a transmission band and a reception band respectively. The latter system switches over the transmission/reception using a filter, a relay and a semiconductor switch.
As the full duplex system needs both transmission and reception frequency bands, this system is not advantageous in comparison with the latter system in consideration of effective use of frequency. However, it has many advantages in its configuration; e.g., it does not need transmission/reception switching control, and the system throughput can be raised. However, if this system is applied to a communication apparatus which can raise the system capability by using a wide band, such as a spread spectrum modulation communication apparatus, the apparatus should use an extremely wide frequency band in order to have dedicated transmission and reception bands. Thus it cannot attain the effective frequency use.
Accordingly, the system as shown in FIG. 53 performs the half-duplex communication, and it has the ANTSW 462 for switching over transmission/reception with the switchover signal S.sub.11.
FIG. 58 is a block diagram showing the half-duplex communication highlighting the switching over of the ANTSW 462.
Half-duplex communication using a common antenna depends on separation of transmission from reception by the ANTSW 463. In order to prevent receiver 1000 from directly receiving a transmission signal, a high-frequency relay having excellent isolation is employed.
FIG. 59 is a block diagram showing the configuration of a SS communication apparatus using a high-frequency relay as an ANTSW. The construction shown in FIG. 59 corresponds to that in FIG. 53 except high-frequency relay 480 and AGC control by amplifier 447 in place of the damping rate control by the ATT 463. Therefore, the correspondent elements have the same reference numerals and the explanations of these elements will be omitted.
In FIG. 59, the high-frequency relay 480 can provide a 40 dB to 50 dB transmission/reception isolation with respect to a 2 GH.sub.z to 4 GH.sub.z frequency.
FIG. 60 is a block diagram showing the configuration of a SS communication apparatus using semiconductor switch 481 instead of the high-frequency relay 480. Similarly to FIG. 59, the elements corresponding to those in FIG. 53 have the same reference numerals and the explanations of these elements will be omitted.
In the apparatus of FIG. 60, a transmission/reception isolation can be about 30 dB with respect to about 2 GH.sub.z frequency. Power supply to the RF AMP 443 and the IF AMP 447 in the receiver is halted by switchover signal S.sub.13 to ensure transmission isolation.
In the apparatus of FIG. 59, because the high-frequency relay 480 is very expensive and the device size is large, downsizing of the system and cost reducing are difficult. Further, response period to the switchover signal S.sub.11 is long, i.e., the interval from the applying of the switchover signal S.sub.11 to normal relay operation is several seconds, thus lowering the system throughput.
When intense external interference wave arrives within the band, the interference wave intrudes in the receiver during transmission. The receiver amplifies the received signal including the interference wave, and the noise exists in the correlation output. As a result, erroneous operations such as generating a carrier sense signal may occur.
FIG. 61 shows signal waveforms respectively representing correlation output, demodulation output, detection output, AGC voltage and a carrier sense signal in case where interference wave has intruded in the receiver.
In the apparatus of FIG. 60, a conceivable problem occurs after the power supply to the RF AMP 443 and the IF AMP 447 is turned off and on again. When this occurs, operations of these AMP's become unstable, and it takes a long period to resume stable amplification operation. Thus, the throughput of the apparatus is extremely lowered.
Further, although the SS modulation communication is generally considered "tolerant of interference waves", if a radiowave transmitted in an intense output exceeding the anti-interference wave capability exists within the band, the apparatus is ill effected, causing degradation of communication quality and communication line disconnection.
Such interference waves cannot be predicted. Therefore, users cannot judge whether the cause of degradation of communication quality is the interference wave or not.
In present situations where using radiowaves in the same frequency band for different purposes has been considered, the above-described influences of the interference waves given to the SS communication apparatuses will be more serious.